1. Field of the Invention
The present invention relates to photo-detecting apparatuses and photo-detecting methods using a photodiode and, in particular, to a photo-detecting apparatus and photo-detecting method for detecting the intensity of weak light with high accuracy.
2. Description of the Related Art
In recent years, with the progress of multimedia networks, demands for communication traffics have been dramatically increasing. For this reason, Wavelength Division Multiplexing (WDM) transmission systems for multi-step relay amplification of optical signals by using an optical amplifier serve an important role in economizing communication systems in multimedia society.
A transmission distance assumed in a WDM transmission system tends to be increased with technical advances of WDM transmission systems, and currently is on the order of 35 decibels at maximum. This means that, for example, an input power of an optical amplifier is extremely small, such as −34 decibels with reference to one milliwatt [dBm] even when an output from its upstream optical amplifier is 1 decibel with reference to one milliwatt.
As depicted in FIG. 7, an optical amplifier 1 generally senses an input power and, with that power, a gain of an Er-doped fiber (EDF) incorporated in the optical amplifier 1 or an attenuation of a Variable Optical Attenuator (VOA) 3 is determined. Therefore, even a small optical power is required to be detected with high accuracy of ±0.1 decibel [dB].
As for branch ratio of an optical demultiplexer 2 depicted in FIG. 2, the influence of a dark current of the photodiode can be more reduced as more power is branched to a photo-detecting apparatus 10 side. Then, however, the light power of a main signal system is decreased, and an optical signal-to-noise ratio (OSNR) after amplification by the optical amplifier 1 is significantly degraded. For this reason, an optical demultiplexer with a branch ratio on the order of 13 decibels down has to be used. At this time, the light-receiving power of the photo-detecting apparatus is extremely small, −47 decibels referred to one milliwatt, which is within a range where the influence of the dark current is not negligible.
If a dark current included in the current to be detected by the photodiode is identified and only that dark current can be corrected, even a small light power can be detected with high accuracy. However, the range of environmental temperature where the optical amplifier is placed is from 0 degree Celsius to 65 degrees Celsius, and is arbitrarily changed depending on the difference in temperature between day and night in one day or the season. The dark current has an extremely high temperature-dependency, and is changed approximately ten folds as the temperature increases within the temperature range. Thus, correction has to be made also in consideration of such temperature characteristics of the dark current.
To simply know an instantaneous dark current, several methods are known, such as a method of temporarily interrupting light power entering the photodiode to measure a dark current and then correcting its amount (refer to Japanese Patent Application Laid-open Publication No. H08-278110, for example) and a method of correcting a dark current by using an auxiliary light-receiving element that generates only a dark current that depends on an ambient temperature with incident light being interrupted (refer to Japanese Patent Application Laid-open Publication No. H08-181348, for example).
However, since it is required to guarantee that the optical amplifier always operates at a predetermined output level in an optical transmission system, the input power is required to be constantly monitored. For this reason, the method of interrupting incident light to directly measure the dark current cannot be applied to the photo-detecting apparatus of the optical amplifier in the optical transmission system.
Moreover, in the method of using an auxiliary light-receiving element, a problem arises such that cost is increased due to the addition of a photodiode. Similarly, also in a method of preparing a thermometer and calculating and correcting a dark current from a relational expression of a dark current to temperature characteristics prepared in advance, a problem arises such that cost is increased due to the addition of a thermometer.
Thus, a challenge is to accurately correct a dark current without increasing cost while always accurately monitoring light power even when a change in environment temperature is abruptly changed.